Embodiments of the inventive subject matter may relate to a powered system, such as a marine vessel and/or off-highway vehicle and, more particularly, to an improved power generation apparatus or system for use with the powered system.
Marine propulsion systems may include one or more internal combustion engines for propelling the vessel by turning a shaft that rotates a propeller. The engines also may supply power to a generator to supply the vessel's electrical loads, for example, for auxiliary drives, which in turn convert electrical power to mechanical power, electrical devices for producing heat, cooling, and light, electrical devices for media technology, and electrical devices for other nautical systems.
Current marine propulsion systems (and off-highway vehicle propulsion systems) employ one or more constant-speed engines to produce turning-torque, or providing rotational forces, for an AC alternator. The alternator generates constant AC current, or frequency, and supplies it to an AC bus. The bus supplies power to motor loads on the marine vessel, including propeller motor drives, thrust motors drives, and other motor drives. The bus also supplies electricity to various hotel loads, i.e., other electrical loads (non-motor loads) that are operational on the marine vessel. Transformers for raising or lowering the AC voltage or for isolation, as required by the motor and non-motor loads, may be interposed between the alternator and the AC bus.
The marine propulsion system may include multiple parallel-connected alternators (driven independently by diesel-powered or other internal combustion engines) feeding a common AC electric bus. However, the alternator outputs must be in-phase, requiring the alternators to be synchronized. Also, reactive AC current (the out-of-phase portion of the AC wave) does little useful work. Such reactive power is pervasive in conventional marine power plants, resulting in reduced voltage levels, the undesirable heating of equipment and wires, and wasted energy. Maintaining stability of this AC system is complex; all alternators must remain in phase. Alternator stability issues include hunting, maximum power-pullout angle, effects of faults, out-of-phase transfers, and load transients. Also, the constant-speed engines may exhibit relatively poorer fuel efficiency due to the required constant speed operation.
FIG. 1 illustrates a prior art configuration showing first and second engines 12 and 14, each supplying rotational energy to a respective alternator 18 and 20. Two transformers 24 and 26 are fed electricity by the respective alternators 18 and 20 at a first voltage and supply electricity at a second voltage to an AC bus 30. Various motor load drives 32, 34 and 36 and a non-motor load 38 are connected to the AC bus 30 as illustrated. The configuration shows the alternator 18/transformer 24 and the alternator 20/transformer 26 in parallel; each the output signals from the transformer 24 and the transformer 26 must operate at the same voltage and at the same frequency. Alternately, the transformers 24 and 26 could be combined into one. Because of this need for synchronization of the alternators, creating this condition requires both the engines 12 and 14 to operate at the same speed.
Another system includes a DC bus and an active front end (a controlled rectifier) to convert the AC to DC for supplying electricity to the DC bus. This system is illustrated in FIG. 2 with active front ends (AFE's) 40 and 42 supplying current to a DC bus 46. The bus 46 supplies DC electricity to DC motors on the marine vessel, including propeller motors 50, thrust motors 52, and other motors 54. The bus 46 is connected to and supplies electricity to other loads and hotel loads 56.
The active front ends 40 and 42 comprise IGBTs used as a rectifier (such as in an IGBT bridge), rather than a diodes used in a diode bridge. The term “IGBT” refers to an insulated gate bipolar transistor. The complexity and, therefore, the cost, of the AFE's 40 and 42 are relatively high. The IGBTs rectify the AC supply to produce the current for the DC bus 46. The IGBT bridge is supplied via a three phase reactor and the IGBTs are controlled with a pulse width modulation waveform such that a sinusoidal current is drawn from the supply. When a drive is regenerating, the DC bus voltage is regulated by the active front end, and the power flows to other loads which are consuming electrical power. In both cases, the speed of the engines 12 and 14 cannot be varied to attain better fuel efficiency.
It may be desirable to have a propulsion system that has different characteristics than those of the currently available systems to improve fuel efficiency while also minimizing part count and overall complexity.